The need is felt of measuring the flowrate of a fluid that flows within pipes of home and industrial fluid distribution networks. Such measurements are particularly important for controlling networks such as aqueducts. In particular, these measurements serve for watching single users' and communities water consumption rate. Moreover, such measurements are useful for detecting fluid leakage from the network.
Monitoring an extended and complex network such as an aqueduct requires the installation of a large number of flowrate meters. Therefore, low cost meters are preferred. Furthermore, it is important that these instruments can be easily and quickly mounted, inspected and replaced. In particular, it is important that these instruments can be inspected and replaced without stopping the fluid flow within the pipes.
In the case of extended networks, the sensors that are installed far from any electric supply network are preferably battery-supplied sensors. In this case, the sensors should preferably be low-consumption sensors, so that the batteries can work for a very long time, for example for some years.
Most of the well-known instruments provide a reliable measure in a relatively narrow measurement range. However, many well-known instruments cannot be used for flowrate measurements in aqueducts, for instance, this is the case of rotameters. Instruments of some well-known types cause a usually intolerable pressure drop, as in the case of about all the volumetric sensors and of the turbine sensors. Less protruding devices are also available, such as the thermal sensors and rotating blade sensors. This kind of sensors provides however a less accurate measurement and requires a very careful installation, which is therefore a critical factor. More accurate and more reliable devices are also known, such as the magnetic sensors and the Doppler sensors. However, due to the physical principle on which the measure is based, energy consumption is higher, and they are normally too expensive for a generalized installation in extended networks such as aqueducts.
Furthermore, most commercially available instruments must be mounted to a nozzle that is hydraulically connected with the duct. Therefore, the fluid flow must be stopped for installing such instruments.
Instruments are also known that allow measuring the flowrate by to measuring the elastic deformation of a probe that is cantilevered within the duct and is in contact with the flowing fluid. The elastic deformation is caused by a fluid dynamic thrust that the flowing fluid applies on the probe. This force, and then the deformation, depends upon the density of the fluid, upon the square of its speed and upon the wet surface area, and also upon the shape of the wet surface. Such devices are described, for instance, in GB 1,252,433 and in GB 830,211, and comprise a means for detecting the deformation, for example a strain gauge i.e. resistance strain gauges, which are arranged on the surface of the probe. The construction of these devices is relatively easy and cheap. However, the electric components that are used for measuring the deformation, which can extend on a relevant portion of the wet surface, must be suitably protected from the fluid. The protection means is made by a not conductive and electrically inert resin or ceramic sheath, or by a layer of a fluid- and preferably heat-resistant material. The protection means should also not reduce the elastic compliance of the probe. Therefore, the design and the construction of these devices is complicated by the coating and by the high temperature sensitivity of the probe. Furthermore, this may limit the application field, i.e. the working temperature range and the fluid type range. In any case, a probe comprising electrical component dipped into a fluid cannot ensure long-time reliability of such a measurement device, since the liquid may leak and stagnate under the coating in a short or long time. Therefore, such sensors are not suitable for installation in remote or somewhat impervious places, as it is often the case for aqueducts nor are they suitable if the fluid contains suspended solids, even occasionally.
Furthermore, when the fluid dynamic thrust ceases, the flowrate measurement devices based on such sensors as resistance strain gauges revert to a prefixed configuration that corresponds to a no-flow condition, i.e. they revert to a “zero” condition, after a significant delay.
Even improved versions of this type of instruments, as described in CN2859465 and U.S. Pat. No. 3,340,733, do not provide any solution to these problems.
Moreover, from US2007/0114023 a device is known which provides using a load cell that is arranged outside of the duct and that is provided with a mechanical connection with a probe that is immersed in the fluid and is movably arranged under the action of the flux. Such devices have the drawback of requiring a seal means that may break, in particular, in the case of remote installations. Accordingly, a leakage of the fluid is possible into the space containing the load cell, which may cause the load cell to be corroded or otherwise damaged.
Furthermore, the flowrate sensors that are based on the measurement of the force exerted by the flowing fluid on an immersed probe provide raw flowrate signals that are affected by the speed profile shape of the fluid flowing within the duct, at the detection point. This requires a process means for processing the raw signal provided by the load cell, which is able to integrate the hydro-dynamic thrust applied along the probe, and/or is able to compare the measure with a reference profile, and to correct it by suitable coefficients, in order to take into account the true distribution of the hydrodynamic thrust, and in order to take into account the deviation between this true distribution and the ideal distribution of the reference profile. This further complicates the construction of such force measurement-based devices.
US2006/248961 discloses a flow detection unit, i.e. a flow switch, for mounting to a duct of a sprinkler plant. The detection unit comprises a support that is adapted to move in case of movement of water within the duct. The support bears a sensor element that is adapted to detect a movement of the support, and a detection circuit is connected to the sensor, in order to produce a detection signal when the sensor element detects a movement of the support caused by a water movement within the duct. A signal processing circuit is also provided that is adapted to produce an alarm signal when the detection signal shows specific features. The support normally comprises a flexible blade arranged within the duct, and the sensor element is normally a load cell. In an exemplary embodiment, the load cell is arranged outside of the duct. In this case, the blade and the load cell are mounted to an inner face and to an outer face of a base element, respectively, i.e. of a flexible planar element that is mounted in a hole made through the wall of the duct. When a sprinkler head opens, the pressure within the tube drops suddenly, and the plane element bends towards within the duct, in such a way that the load cell experiences a deformation and produces a signal of the pressure drop. Such device has the drawback of not bearing a large number of deformation cycles of the flexible plane element, as it would occur in an attempt to use the device to as a flowrate measurement device, without damaging the flexible plane element and jeopardizing the seal capability and/or the operation of the load cell.
WO 99/50621 describes a flowrate sensor for installation in a duct of a drop-irrigation system comprising a drag blade that is pivotally arranged in the flow within the duct, and that is equipped with a first magnet, in such a way that it can move upwards and downwards responsive to the flowrate of the fluid. An indication lever is pivotally arranged outside of the duct, and is provided with a second magnet that is arranged to interact with the first one. This way, the external lever can arrange itself according to the movement of the internal blade. A position-adjustable counterweight is arranged on the lever, so that the lever is aligned with a given reference line to indicate a given flowrate value of the fluid within the duct.